1. Field of the Invention
This invention relates to an apparatus and method for determining the state of penetration of the weld pool and more particularly to an apparatus and method utilizing a photo-position detector to monitor the angular position of a beam of light specularly reflected from the weld pool to determine the state of penetration.
2. Description of the Prior Art
Several techniques have been disclosed for determining the state of penetration of a weld pool. One technique disclosed in "Automatic Control of Weld Penetration" by D. C. Buffum, Army Materials & Mechanics Research Center Final Report No. AMMRC-TR-78-6, Feb. 1978, involves placing a sensor (optical, infrared, etc.) on the root side of the weld joint to detect whether or not full penetration has been achieved. The major problem with this technique is that it requires accessibility to the root side of the joint. Adequate accessibility is not available on the hardware to be welded.
Another technique that has been applied to weld process monitoring is infrared thermography of the weld pool ("Infrared Thermography as a Control for the Welding Process" by M. A. Kahn, N. H. Madsen and B. A. Chin, SPIE, Vol, 446, pp. 154-165, SPIE 1983). In this procedure, the isotherms in and around the weld pool are monitored by infrared detectors. If sufficient experimental work is done, the isotherm distribution observed at the arc surface of the weld can be correlated with level of penetration. However, several aspects of this approach are unfavorable. First, very slight variations in the heat sink properites of the hardware being welded can significantly alter the isotherm pattern. Second, accurate calibration of the system requires a knowledge of the emissivity of the material's surface. This property will vary with surface finish, thermal history and surface cleanliness. Finally, determination of penetration by this technique requires a reliable model of the temperature profile of the weld pool. These profiles can vary greatly with only minute differences in alloy composition.
Another scheme for penetration monitoring depends on the principal that arc voltage is a function of arc length. In work disclosed in "Experimental Investigation of GTA Weld Pool Oscillations" by R. J. Renwich and R. W. Richardson, Welding Research, Supplement to the Welding Journal, pp. 29-S - 35-S., Feb. 1983, the welding arc current was transiently pulsed from the normal value to a higher value. This current change caused a sudden change in the magnetic constrictive force on the arc and, consequently, a change in the velocity of gas in the arc. As a result, there was a sudden pressure impulse on the weld pool surface as gas velocity changed. This pressure impulse initiated a resonant oscillation of the weld pool surface. As the surface oscillated, the distance between it and the electrode tip, i.e. the arc length, varied and was detected as a change in arc voltage. Therefore, monitoring arc voltage permitted observation of the weld pool natural frequency. (The natural frequency is a function of the weld pool's dimensions, surface tension and mass.) The experimental results showed a resonably good correlation between frequency and the inverse of the square root of weld pool mass. The method disclosed by Renwick and Richardson allows a reasonable measurement of puddle mass and furthermore puddle width can be determined optically. However, the method requires a model that relates puddle mass, puddle width, and puddle depth and no such reliable model exists. Therefore, the method provides only an indirect, inferential and uncertain means of measuring weld penetration.
U.S. Pat. No. 4,491,719 entitled, "Light Pattern Projector Especially for Welding" issued to N. R. Corby, Jr. utilizes a light pattern which is projected onto the surface of a weld pool. An image of the pattern is impressed on the image of the pool due to diffuse scattering at the pool surface. A camera is focused on the weld pool yielding information on weld pool topography from which weld quality can be inferred. The system relies on the fact that the projected pattern will appear on the image on the pool surface and thus relies on the non-specular quality of the pool surface. However, in many welding processes a highly specular weld pool can be expected.